I've experimented with the Ruger oversized "Target" grips, the Ruger "Splinter" grips, the Hogue rubber fingergroove grip, and Trausch TJ84R grips (originally for a round-butt Speed/Service Six, but modified for me by the late Jacques Trausch via some strategic cut-outs to accommodate the Security Six's square butt. The Trausch grips are superb, providing excellent autopistol ergos to a revolver is the best way I can describe them. Although ugly, Jacques further modified a set for me by whittling a crescent speedloader cut-out on the left grip on a set.
Now for the unfortunate part-they were virtually unknown over here, and Jacques never really effectively linked up a viable distributor in the US, so they're pretty much made of unobtanium-but definitely worth the effort to search out. If you find a set, let me know, and I'll send you some detailed images of mine, so you'll know where to make the requisite cut-outs.
A pretty decent second choice would be the Hogues.
Some images:
With left grip speedloader cut-out:
Best, Jon